Planctomycetes

Planctomycetes are an order of aquatic bacteria and are found in field samples of brackish, and marine and fresh water samples. They reproduce by budding. In structure, the organisms of this group are ovoid and have a holdfast, called the stalk, at the nonreproductive end that helps them to attach to each other during budding.

Cavalier-Smith has postulated that the Planctomycetes are within the clade Planctobacteria in the larger clade Gracilicutes.

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Structure

The organisms belonging to this group lack murein in their cell wall. Murein is an important heteropolymer present in most bacterial cell walls that serves as a protective component in the cell wall skeleton. Instead their walls are made up of glycoprotein rich in glutamate. Planctomycetes have internal structures that are more complex than would be typically expected in prokaryotes. While they don't have a nucleus in the eukaryotic sense, the nuclear material can sometimes be enclosed in a double membrane. In addition to this nucleoid, there are two other membrane-separated compartments; the pirellulosome or riboplasm, which contains the ribosome and related proteins, and the ribosome-free paryphoplasm (Glockner, 2003).

Genome

RNA sequencing shows that the planctomycetes are distantly related to the other eubacteria. A number of essential pathways are not organised as operons, which is unusual for bacteria (Glöckner, 2003). A number of genes have been found (through sequence comparisons) that are similar to genes found in eukaryotes. One such example is a gene sequence (in Gemmata obscuriglobus) that was found to have significant homology to the integrin alpha-V, a protein that is important in transmembrane signal transduction in eukaryotes (Jenkins et al., 2002).

Life cycle

The life cycle involves alternation between sessile cells and flagellated swarmer cells. The sessile cells bud to form the flagellated swarmer cells which swim for a while before settling down to attach and begin reproduction.

References

• F. O. Glöckner, M. Kube, M. Bauer, H. Teeling, T. Lombardot, W. Ludwig, D. Gade, A. Beck, K. Borzym, K. Heitmann, R. Rabus, H. Schlesner, R. Amann, and R. Reinhardt (2003). Complete genome sequence of the marine planctomycete Pirellula sp. strain 1. PNAS 100 (14): 8298–8303.
• Cheryl Jenkins, Vishram Kedar, and John A. Fuerst (2002). Gene discovery within the planctomycete division of the domain Bacteria. Genome Biology 3 (6): research0031.1–0031.11.

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